Method for manufacturing a panel for reflective broadband electromagnetic shield

ABSTRACT

A panel for an electromagnetic shield includes a light-weight, porous, electrically-conductive core layer of metallic foam having generally parallel opposed surfaces and a face sheet having rigidity properties superior to the rigidity properties of the core layer laminated to a surface of the core layer. Alternatively, a panel for a broadband electromagnetic shield includes a composite fiber-reinforced core having opposed surfaces and a layered electrically-conductive composite cover disposed on a surface of the core. The cover includes a first stratum of porous metal exhibiting pronounced low-frequency electromagnetic shielding properties and a second stratum of electrically-conductive elements exhibiting pronounced high-frequency electromagnetic shielding properties secured in an overlapping electrically-continuous relationship to the first stratum, the first stratum being a metallic lattice, and the electrically-conductive elements being a non-woven veil of electrically-nonconductive metal-coated fibers.

CROSS-REFERENCED RELATED APPLICATIONS

This application claims the benefit of and is a continuation-in-partapplication of U.S. Provisional Application Ser. No. 61/262,386 that wasfiled on Nov. 18, 2009. This application is also a continuation-in-partapplication of U.S. patent application Ser. No. 12/698,961 that wasfiled on Feb. 2, 2010, now U.S. Pat. No. 8,415,568 claiming the benefitas a continuation-in-part application of U.S. Provisional ApplicationSer. No. 61/149,116 that was filed on Feb. 2, 2009.

This application is related by subject matter to each of the followingpending United States patent applications, which are individually andcollectively incorporated herein by reference:

-   -   a. U.S. patent application Ser. No. 10/414,266;    -   b. U.S. patent application Ser. No. 11/609,113;    -   c. U.S. patent application Ser. No. 12/260,999; and    -   d. U.S. patent application Ser. No. 12/261,006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the design of electromagneticshielding. More particularly, the present invention pertains to panelscapable of providing electromagnetic shielding for an interior space.

2. Background

Optimum electromagnetic shielding offers broadband protection from verylow frequencies on the order of a few kilohertz to very high frequencieson the order of tens of gigahertz. Electromagnetic signals at lowfrequencies have long wavelengths, while electromagnetic signals of highfrequencies exhibit short wavelengths. Signals of different wavelengthsinteract differently with any electromagnetic-shielding material.

Fundamental material properties impact the capability of a material toshield from electromagnetic signals. The most significant of these areelectrical conductivity, magnetic permeability, thickness, and geometricmorphology. The most relevant of these material variables is dictated bythe situation of use, as for example, by the intensity of anticipatedelectromagnetic radiation, or by whether the radiation is emitted from aproximate near field source or a remote far domain source.

Polymer systems and fiber-reinforced composite materials play anincreasingly important structural role in many contemporaryapplications. These materials exhibit high specific stiffness andstrength. Many are advantageously easy to manufacture and assemble.Still, as a group such polymer and composite systems exhibit poorelectrical properties, and are not, therefore, particularly recommendedfor use in electromagnetic shielding.

BRIEF SUMMARY OF THE INVENTION

According to an initial aspect of the present invention, a panel for anelectromagnetic shield includes a light-weight, porous,electrically-conductive core layer defined between generally parallelfirst and second surfaces and a first face sheet laminated to the firstsurface of the core layer and having rigidity properties superior to therigidity properties of the core layer. The core layer may be made of afluid-permeable material. The distance between the first and secondsurfaces of the core layer defines a thickness of the core layer. Thethickness of the first face sheet may substantially less than thethickness of the core layer, approximately equal to that of the corelayer, or even larger that the thickness of the core layer.

In one embodiment, the core layer is made of metallic foam. Anotherembodiment includes a metal coating on an electrically-nonconductive,porous, nonmetallic substrate chosen from a group of materials thatincludes nonwoven fibrous matting, paper, and open-cell nonmetallicfoam. Alternatively, the core layer is made up of liberated branchingmetal nanostrands or a plurality of electrically-coupled,electrically-conductive particles. Each of the particles takes the formof an electrically-nonconductive, nonmetallic substrate with a metalcoating.

The first face sheet includes a cured layer of resin withelectrically-conductive elements distributed throughout the resin. Theelectrically-conductive elements are selected from among a groupincluding liberated branched metal nanostrands, metal wires, and metalmeshes, or electrically-conductive elements in the first face sheet areselected from a group of electrically-conductive elements aremetal-coated fibers, woven fabric, nonwoven matting, or paper. Bothbeing electrically-conductive, between the first face sheet and thefirst surface of the core is interposed an electrically-conductiveadhesive layer.

The inventive panel may also include a fluid-impermeable second facesheet that is laminated to the second surface of the core layer. Thesecond face sheet has rigidity properties superior to the rigidityproperties of the core layer. When the second face sheet iselectrically-conductive, and the panel also includes anelectrically-conductive adhesive layer between the second face sheet andthe second surface of the core layer.

In yet another aspect of the present invention, a panel for a broadbandelectromagnetic shield includes a core having first and second surfaceson opposite sides thereof with the distance between the first and secondsurfaces defining the thickness of the core. A layeredelectrically-conductive composite cover is disposed on the first surfaceof the core having a thickness substantially less than the thickness ofthe core. The cover contains a first stratum of porous metal secured tothe core and exhibiting pronounced low-frequency electromagneticshielding properties and a second stratum of electrically-conductiveelements exhibiting pronounced high-frequency electromagnetic shieldingproperties secured in an overlapping electrically-continuousrelationship to the first stratum. An adhesive may be used to secure thefirst stratum to the first face of the core, or the first stratum may besecured to the core indirectly by way of the second stratum.

The first stratum is a metallic lattice of the type chosen from a groupcomprising a metal screen, a metal mesh, an expanded metal foil, and aperforated metal sheet. Alternatively, the first stratum may take theform of a noncontinuous, patterned metal film disposed on the firstsurface of the core using a process selected from the group comprisingspraying, vapor deposition, electrodeposition, dipping, rolling, andbrushing. The first stratum is made from a metal, such as a μ-metal,nickel, copper, aluminum, bronze, iron, steel, and alloys of anythereof. An adhesive may used to secure the second stratum to the firstface of the core, or the second stratum may be secured to the coreindirectly by way of the first stratum.

The electrically-conductive elements of the second stratum are anon-woven veil of electrically-nonconductive fibers coated by a metal.The fibers assume the form of a scrim, a mat, and a paper ofindividually-metalized fibers or fibers assembled in a non-wovenrelationship and coated collectively by a metal in a chemical vapordeposition. The metal used in the second stratum may be nickel, copper,aluminum, iron, and alloys of any thereof.

The core may or may not be structurally self supporting, orsubstantially planar with parallel first and second faces. The core maybe made entirely of an electrically nonconductive resin, or the core mayin the alternative be made a resin body with fibers of glass, carbon, ora polymer embedded therein.

The teachings of present invention also include various relatedmethodologies. For example, within the scope of the present inventionare methods for manufacturing a panel for an electromagnetic shield.Also within the scope of the present invention are methods formanufacturing an electromagnetic shielding cover for composite panels ofdiverse constructions constructions.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In order that the manner in which the above-recited and other featuresand advantages of the present invention are obtained will be readilyunderstood, a more particular description of the present inventionbriefly described above will be rendered by reference to specificembodiments thereof which are illustrated in the appended drawings.Understanding that these drawings depict only typical embodiments of thepresent invention and are not therefore to be considered to be limitingof scope thereof, the present invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is an illustrative depiction of a source of electromagneticsignal radiation and an enclosure for an interior space that, byincorporating teachings of the present invention, provides broadbandelectromagnetic shielding for that interior space;

FIG. 2 is a cross section of the enclosure of FIG. 1 taken along sectionline 2-2 therein that includes in an inset a highly magnified view incross section of a first embodiment of a panel of an electromagneticshield incorporating teachings of the present invention;

FIG. 3 is a graph depicting the effectiveness of foam metal as a core ofa composite electromagnetic shielding panel;

FIG. 4 is a cross section like that of the inset of FIG. 2 providing ahighly magnified view in cross section of a second embodiment of a panelof an electromagnetic shield incorporating teachings of the presentinvention;

FIG. 5 is a graph depicting the electromagnetic shielding effectivenessof a layer of expanded metal of the type employed in the cover of thepanel of the electromagnetic shield illustrated in FIG. 4;

FIG. 6 is a graph depicting the electromagnetic shielding effectivenessof chemical-vapor-deposition metal-coated non-woven fibers of the typeemployed in the cover of the panel of then electromagnetic shielddepicted in FIG. 4;

FIG. 7 is a graph depicting generally typical electromagnetic shieldingeffectiveness of the layers in the cover of the panel of theelectromagnetic shield depicted in FIG. 4; and

FIG. 8 is a graph depicting the comparative electromagnetic shieldingeffectiveness of the metallic materials employed in the embodiments ofelectromagnetic shielding panels depicted, respectively, in FIGS. 2 and4.

DETAILED DESCRIPTION OF THE INVENTION

The presently preferred embodiments of the present invention will bebest understood by reference to the drawings, wherein like parts aredesignated by like numerals throughout. It will be readily understoodthat the components of the present invention, as generally described andillustrated in the figures herein, could be arranged and designed in awide variety of different configurations. Thus, the following moredetailed description of the embodiments of the present invention, asrepresented in FIGS. 1-4, is not intended to limit the scope of theinvention, as claimed, but is merely representative of presentlypreferred embodiments of the invention.

FIG. 1 depicts a typical environment in which teachings of the presentinvention find utility. There a radio transmitter tower 10 is shown as atypical source of outgoing electromagnetic signal radiation 12 thattravels outwardly away from radio transmitter tower 10, as suggested byarrows E. Shielding from electromagnetic signal radiation 12 is desiredon behalf of a sensitive electronic device within the transmission rangeof transmitter tower 10.

Toward that end, the electronic device, which is not actually visible inFIG. 1, is housed in an electromagnetic-shielding enclosure 20 thatincorporates teachings of the present invention.Electromagnetic-shielding enclosure 20 thus provides broadbandelectromagnetic shielding from electromagnetic signal radiation 12 forthe interior space within electromagnetic-shielding enclosure 20. Asseen by way of example in FIG. 1, electromagnetic-shielding enclosure 20includes a square electromagnetic-shielding front panel 22, arectangular electromagnetic-shielding top panel 24, and a similarlyshaped electromagnetic-shielding side panel 26. Corresponding oppositepanels (not visible in FIG. 1) are disposed opposite and spaced frompanels 22, 24, and 26 to form the enclosure 20. In FIG. 2, the panelopposite top panel 24 is floor panel 44 and the panel opposite sidepanel 26 is second side panel 42. Panels 22, 24, 26 and opposite panelsmeet and are attached in an electrically coupled manner at respectivejoints 28 between each pair thereof. As a result, electromagnetic signalradiation 12 is precluded from penetrating into the interior of theenclosure 20 through any of the electromagnetic shielding panels orinterfering with the intended operation of the electronic device housedwithin the enclosure.

An electromagnetic-shielding enclosure, such aselectromagnetic-shielding enclosure 20, may assume diverseconfigurations, involving assemblies with greater or lesser symmetry,with irregular or nonplanar outer surfaces, with more or fewer panelsand more or fewer joints therebetween than are included inelectromagnetic-shielding enclosure 20. For example, anelectromagnetic-shielding enclosure, such as electromagnetic-shieldingenclosure 20, may take the form of a pair of semisphericalelectromagnetic-shielding panels that are attached in an electricallycoupled manner at a single continuous circumferential joint. As usedherein and applied to an electromagnetic-shielding panel, theexpressions planar and substantially planar are intended to include evenelectromagnetic-shielding panels that, while exhibiting curvatures orhaving flexibility, have a thickness between the opposite surfacesthereof that is substantially unchanging throughout the entireelectromagnetic-shielding panel.

The physical and material considerations governing the formation ofpanels 22, 24, and 26 will be addressed following a brief discussion ofsome material properties that impact the capability of the constituentmaterials of panels 22, 24, and 26 to effectively shield the interior ofelectromagnetic-shielding enclosure 20 from electromagnetic signals 12.

One characteristic of electromagnetic waves is wave impedance. This isdefined as the ratio of the electric field (E-field, volts per meter) tothe magnetic field (H-field, amps per meter) of the electromagneticradiation. A low impedance wave will have a low E-field and highH-field, and a high impedance wave will have a high E-field and lowH-field. Thus, low impedance wave shielding is concerned more withmagnetic permeability and absorption, while high impedance waveshielding is concerned more with electrical conductivity.

The shielding effectiveness SE of an electromagnetic-shielding materialis expressed in decibel units, and represents the quantity of anelectromagnetic field that is attenuated by an electromagnetic-shieldingbarrier made of the electromagnetic-shielding material in question.

The shielding effectiveness SE of a material is calculated usingEquation 1 below.SE−R+A+B,  Equation 1:

-   -   where, R=reflection shielding mechanism in decibels;        -   A=absorption shielding mechanism in decibels; and        -   B=re-reflection shielding mechanism in decibels.

The reflection shielding mechanism R of an electromagnetic-shieldingmaterial is expressed in decibel units. Reflection shielding mechanism Ris a function variously of (1) the impedance Z₁ of anelectromagnetic-shielding wave in a medium surrounding anelectromagnetic-shielding barrier made of the electromagnetic-shieldingmaterial in question, (2) the impedance Z₂ of theelectromagnetic-shielding wave in the electromagnetic-shieldingshielding barrier, (3) the electrical conductivity σ of theelectromagnetic-shielding barrier, (4) the magnetic permeability μ ofthe electromagnetic-shielding barrier, and (5) the frequency f of theelectromagnetic-shielding wave itself.

The impedance Z₂ of an electromagnetic-shielding wave in anelectromagnetic-shielding shielding barrier is calculated using Equation2 below

$\begin{matrix}{{Z_{2} = \sqrt{\frac{2\;\pi\; f\;\mu}{\sigma}}},} & {{Equation}\mspace{14mu} 2}\end{matrix}$

-   -   where, f=electromagnetic-shielding wave frequency in hertz;        -   μ=electromagnetic-shielding barrier magnetic permeability in            volt-second per ampere per meter; and        -   σ=electromagnetic-shielding barrier electrical conductivity            in siemens per centimeter.

The reflection shielding mechanism R of an electromagnetic-shieldingmaterial is calculated using Equation 3 below.

$\begin{matrix}{{R = {20\;\log\;\left( \frac{Z_{1}}{4\; Z_{2}} \right)}},} & {{Equation}\mspace{14mu} 3}\end{matrix}$

-   -   where, Z₁=impedance in ohms of medium surrounding        electromagnetic-shielding barrier; and        -   Z₂=impedance in ohms of electromagnetic-shielding shielding            barrier.

The greater the impedance mismatch between an electromagnetic-shieldingshield and its surrounding medium, the greater the shieldingeffectiveness of the electromagnetic-shielding shield. Thus, the use ofhighly electrically-conductive materials leads to greater reflection.

The absorption shielding mechanism A is expressed in decibels and iscalculated using Equation 4 below.A=20 log e _(δ) ^(t),  Equation 4:

-   -   where, t=electromagnetic-shielding barrier thickness in length        units; and        -   δ=electromagnetic-shielding barrier skin depth in length            units.            An increase in the exponent of Equation 4 is achieved by            increasing thickness t or decreasing skin depth δ.

Skin depth δ can be considered to be the depth of penetration into anelectromagnetic-shielding barrier by an incident electromagnetic wave ata given frequency. Fundamentally, skin depth δ dictates the loss ofamplitude of an incident wave in an electromagnetic-shielding barrier.Thus, a given skin depth will lead to a proportional unit loss ofamplitude in any incident electromagnetic wave. For example, a singleunit of skin depth typically reduces the amplitude of an electromagneticwave by 1/e, or about 37%. Thus, reducing the skin depth of anelectromagnetic-shielding material leads to reductions in the totalrequired thickness of an electromagnetic-shielding made from thematerial, while still maintaining electromagnetic shield levels.

Skin depth δ is calculated using Equation 5 below.

$\begin{matrix}{{\delta - \frac{1}{\sqrt{\pi\; f\;\sigma\;\mu}}},} & {{Equation}\mspace{14mu} 5}\end{matrix}$

-   -   where, f=electromagnetic-shielding wave frequency in hertz;        -   σ=electromagnetic-shielding barrier electrical conductivity            in siemens per centimeter; and        -   μ=electromagnetic-shielding barrier magnetic permeability in            volt-second per ampere per meter.            Skin depth δ can be decreased by increasing one or both of            electrical conductivity σ and magnetic permeability μ. Skin            depth naturally decreases with increasing electromagnet wave            frequency f.

Physical and materials considerations of the geometrical morphology ofthe electromagnetic shield can also have importance. A solid, planarelectromagnetic-shielding barrier must rely on its bulk materialproperties of conductivity and permeability alone for shieldingeffectiveness. An improved geometric morphology will be disclosed belowrelative to FIG. 2.

FIG. 2 is a cross-sectional plan view of front panel 22 ofelectromagnetic-shielding enclosure 20 between joint 28 at side panel 26and joint 28 at a second side panel 42 that is not visible in FIG. 1.Also newly revealed in FIG. 2 is a floor panel 44 ofelectromagnetic-shielding enclosure 20 that is provided with the sameelectromagnetic shielding capacity as are the other panels ofelectromagnetic-shielding enclosure 20. Resting upon floor panel 44,interior of electromagnetic-shielding enclosure 20 in the vicinity ofjoint 28 between front panel 22 and second side panel 42, is anelectronic device 46 that is being shielded from electromagnetic signalradiation 12 by electromagnetic-shielding enclosure 20.

Each of the panels of electromagnetic shielding enclosure 20 may be of aunique construction, provided that each panel incorporates appropriateinsights from among the teachings of the present invention. For the sakeof simplicity herein, however, it will be assumed that the constructionof each of the panels of electromagnetic-shielding enclosure 20 isidentical. Thus, the internal construction of side panel 26 will beexplored in significant detail as being typical of all of the panels ofelectromagnetic-shielding enclosure 20, and the same referencecharacters used to do so in the following discussion will be usedwithout further introduction whenever convenient herein and in thesupporting drawings.

Side panel 26 includes a light-weight, porous, electrically-conductiveplanar core layer 50 having a first surface 52 that is orientedoutwardly of electromagnetic-shielding enclosure 20 and a generallyparallel second surface 54 on opposite side of core layer 50 that isoriented inwardly of electromagnetic-shielding enclosure 20. Thedistance between first surface 52 and second surface 54 defines athickness T₅₀ of core layer 50. Laminated to first surface 52 of corelayer 50 is a first face sheet 56, and laminated to second surfaces 54of core layer 50 is a second face sheet 58. The outer surface of firstface sheet 56 thus coincides with outer surface 36 of side panel 26,while the outer surface of second face sheet 58 defines an inner surface60 of side panel 26. Similarly, the outer surface of first face sheet 56coincides with the outer surface 32 of front panel 22 and with the outersurface 34 of top panel 24, respectively.

Either or both of first face sheet 56 and second face sheet 58 haverigidity properties that are superior to the rigidity properties of corelayer 50. As used herein and applied to face sheets, such as first facesheet 56 and second face sheet 58, superior rigidity properties includesuperior tensile properties, superior stiffness properties, or superiortensile and stiffness properties combined. When laminated to core layer50, either of both of first face sheet 56 and second surfaces 54supplement the mechanical strength of core layer 50, causing side panel26 to be able to function as a sturdy structural element of anyenclosure in which it is included. For this reason, in many, but notnecessary in all, occasions, first face sheet 56 has a thickness T₅₆that is substantially less than thickness T₅₀ of core layer 50.Similarly, second face sheet 58 may have a thickness T₅₈ that is alsoless than thickness T₅₀ of core layer 50. By way of example, thicknessT₅₀ of core layer 50 may be in the range of about 0.0625 inches, inwhich case thickness T₅₆ of first face sheet 56 and thickness T₅₈ ofsecond face sheet 58 might be in the range of, or thickness T₅₈ ofsecond face sheet 58 may be approximate to or even greater thanthickness T₅₀ of core layer 50.

Core layer 50 may, to great advantage, be made of metal foam.Alternatively, core layer 50 may include a porous nonmetallic substrateand a metal coating on that substrate. The substrate in core layer 50may be electrically-nonconductive and may be formed from a materialselected from a group of materials comprising nonwoven fibrous matting,paper, and open-cell nonmetallic foam. Core layer 50 may also be made ofa plurality of electrically-coupled, electrically-conductive particles.The electrically-conductive particles may be liberated branching metalnanostrands, or each of the electrically-conductive particles can be anonmetallic substrate with a metal coating. Then, the nonmetallicsubstrate in each of the electrically-conductive particles may itself beelectrically-nonconductive. Quite advantageously, core layer 50 isfluid-permeable in some embodiments.

The use of nickel foam as a core material was unknown prior to thepresent invention. Advantageously, light-weight nickel foam is an itemof commerce, although one available from only a limited number ofsources. Nickel foam core provides the advantages of highly effectivebroadband shielding to a frequency of at least about 20 gigahertz, theadded stiffness of foam core construction, and excellent through-planeand in-plane heat transfer properties, with opportunities, if desired,for both passive and active heat transfer.

Core layer 50 with the assistance of either or both of first face sheet56 and second face sheet 58 need not rely on its own bulk material inorder to serve as a sturdy structural element ofelectromagnetic-shielding enclosure 20. Core layer 50 can take the formof a grating, or screen pattern of discreet wires. Further, core layer50 may have an open pattern built from overlapping interconnected layersof such materials three-dimensionally randomly positioned relative toeach other. Such a pattern of overlapped materials results substantiallyin a physically-obstructed planar barrier to incident electromagneticwaves, such as electromagnetic signal radiation 12, but such a patternof overlapped materials allows electromagnetic waves that do penetratethe outermost layers of the pattern to be reflected internally andabsorbed within the overlapped materials in the manner of a network offaraday cages within a shielding barrier that trap and dissipatepenetrative incident electromagnetic radiation.

FIG. 3 is a graph illustrating the electromagnetic shieldingeffectiveness of foam metal as the core of a panel of an electromagneticfield incorporating teachings of the present invention. There theperformance of a stand-alone metal foam core and of a metal foam corewith a pair of conductive face sheets compare favorably therein to theperformance of a solid sheet of aluminum foil having a thickness of0.005 inches.

Three dimensionally interconnected, open, and conductive porousmaterials can be made by coating with metal any of a number ofsubstrates, including but not limited to foams, felts, fabrics, pads,scrims, papers, fiber matting, and filter media. Porous media coatedwith or made of nickel are particularly appropriate for several reasons.First of all, nickel is highly electrically conductive σ and exhibitshigh magnetic permeability μ. It is seen in the above equations thatthese material properties are advantageous in electromagnetic shielding,both independently and in combination with other constituent componentsof a composite structure. Porous media is additionally advantageous inelectromagnetic-shielding applications, as providing athree-dimensionally interconnected, conductive grating to incomingradiation, along with a multiplicity of interconnected faraday cages.The use of porous media helps to keep weight to a minimum. Additionaladvantages, such as heat transfer through either or both of conductionand convection are afforded with a porous media that isthermally-conductive and possessed of a relatively high heat transfercoefficient. Porous media of several sizes can be combined to increasethe broadband electromagnetic-shielding effectiveness of a composite.The porous media layers also act as structural or core layers incomposite systems.

The key geometry in the above-described materials is the nature of thethree-dimensional porosity. These kinds of materials can also be used ondifferent scales to achieve different regions of shieldingeffectiveness. For example, substrates with smaller pores will shieldhigher frequencies, while substrates with larger pores will shield lowerfrequencies. The use of flexible or compressible porous media, such asnickel coated foam is recommended. With porous foam materials, thecompressibility and flexibility of the foam is paramount. These foammaterials can be face stitched to metal-coated fabric. While, such a drysandwich is highly deformable and easily bendable and can serve as andcreates an excellent electromagnetic-shield material, a dry sandwich ofthis type is not particularly recommended as a structural material.

As discussed above, however, an electromagnetic-shield panel, such asany panel of electromagnetic-shielding enclosure 20, is much morephysically robust than the dry sandwich material discussed above. Theuse of a laminate composite involving core layer 50 among other layersresults in stand-alone shielding layers, such as side panel 26 exploredin relation to FIG. 2. The shielding effectiveness of composite systemsand polymers is improved by using conductive porous materials, eitheralone or as core layers in structural laminates. Several differentmaterials may be combined to produce any specific desired effect.Unprecedented levels of shielding effectiveness in composite materialshave been demonstrated with these conductive porous layers, both aloneand in composite systems. Nickel nanostrands, nickel coated fibers, andnickel coated non-woven paper may also be used in these systems toprovide an additional region of broadband electromagnetic-shieldingeffectiveness.

One aspect of the present invention is that electrically-conductive andthermally-conductive foam cores act as foam core stiffeners in compositelaminates. For instance, while non-conductive foam cores provideoutstanding mechanical stiffness and energy absorption in a compositestructure, the incorporation of metal foam cores provides the samemechanical advantages, but incorporates excellent thermal conductivityand electrical conductivity to the composite. The later property lendselectromagnetic-shielding capacity to the composite.

When electrically-conductive foams are properly incorporated betweenlaminate panels, such as the panels of electromagnetic-shieldingenclosure 20 that are themselves electrically-conductive, the resultingcomposite structure displays an increased effective electromagnetic skindepth relative to the electromagnetic skin depth of the core foammaterial alone material. Thus, an electromagnetic-shielding-panel, suchas side panel 26 of electromagnetic-shielding enclosure 20, mayincorporate face sheets, such as first face sheet 56 and second facesheet 58, that are themselves electrically-conductive. The face sheetmay include a cured layer of resin and electrically-conductive elementsdistributed throughout the layer of resin. Those electrically-conductiveelements may be selected from a group of electrically-conductiveelements comprising liberated branched metal nanostrands, metal wires,and metal meshes. In the alternative, the electrically-conductiveelements are selected from a group of electrically-conductive elementscomprising, fibers, woven fabric, nonwoven matting, and paper. These maybe metal-coated as required by the circumstance of intended use. When anelectrically-conductive face sheet is employed, it is advantageous toemploy an electrically-conductive adhesive layer between the face sheetand the associated surface of the core layer.

A composite panel results that has commendable electrical conductivityand highly effective electromagnetic shielding, but that is yet verylight-weight. For instance, panels with surface resistivity of a fewmilliohms and a volume resistivity equal to or less than a fewmilliohm-cm have been fabricated. These same panels, while being highlyconductive and acting as very electromagnetically thick-skinned shieldsfor low-frequency electromagnetic signals, also exhibit the light weightand high stiffness of foam core laminate composites. Composites with adensity less than 1.00 gm/cc and a stiffness that is approximatelyequivalent to that of aluminum have been produced, and these exhibitelectromagnetic-shielding capabilities that are beyond the limit ofavailable electromagnetic test methodologies.

Thus, the teachings of the present invention include the use of openlight-weight, electrically-conductive, fluid permeable materials asmultifunctional structural core layer material for composite structuresand as stand-alone broadband shielding panels with properties that arein some ways superior to those made from wires, meshes, or honeycombs.Such materials may be metal foam, if open-cell and structurally-stable,or any porous metal structure, whether it be foam, felt, or non-woven.The core may be a metal-coated porous material, such as open-cell orclosed-cell foam, felt, cloth, or fiber matting. The core may also bemade of metal particles, metal-coated particles, or self-supportingmetal nanostructures, such as liberated branching metal nanostrands. Theuse of high electromagnetic-permeable metals, such as nickel,copper-nickel, nickel-iron, and perm alloy is recommended, particularlyfor shielding from lower frequency electromagnetic radiation.

In manufacturing a conductive composite, control is recommended ofelectrically conductivity and the electrical permeability of allconstituent elements of the composite. In light of the series ofequations provided earlier, the goal should be to make the overallcomposite structure as electromagnetically thick as possible. Thesurface, the resin, the reinforcing fibers, and the core adhesiveemployed in the composite should each be as electrically-conductive aspermitted by circumstances of anticipated use. Nickel nanostrands may beemployed to create electrically-conductive surfaces, resins, andadhesives. Nickel-coated paper and nickel-coated fibers are recommendedwhen reinforcement is called for, either in an electrical or in astructural sense. If the electromagnetic conductivity of any componentor phase of a composite is poor, then the performance of the entirecomposite system will be compromised and not optimized.

Nickel foam alone does a great portion of low-frequencyelectromagnetic-shielding. Within nickel foam are air passages thatpermit fluid flow, while the nickel of the foam affords broadbandelectromagnetic shielding. In structural applications, both in rigidpanels made with epoxy composite and in flexible panels made with anelastomeric matrix, greater electromagnetic-shielding is attained wherethe surface and structural element of a composite panel is rendered aselectrically-conductive as possible. Such a comprehensive and specificapproach to electrical conductivity is appropriate, where multiplepanels employing porous core composites are to be joined in anelectrically continuous fashion into a larger overall structure.Adhesives and gap fillers are capable of effecting the desired joiner,if electrically-conductive.

The teaching of present invention also include methodologies. Forexample, within the scope of the present invention are methods formanufacturing a panel for a broadband electromagnetically shield.

By way of example, one embodiment of a method incorporating teachings ofthe present invention for manufacturing a panel for a broadbandelectromagnetic shield involves shaping a light-weight porous,electrically-conductive material into a substantially planar core layer,such as core layer 50, having first and second surfaces on oppositesides thereof. The process of shaping the core layer can be undertaken,either by first producing metal foam and then using the metal foam asthe electrically-conductive material from which to form the core, or byproducing a porous nonmetallic, electrically-nonconductive substrate,and coating the substrate with a metal.

Then a first face sheet, such as first face sheet 56 of FIG. 2, havingrigidity properties superior to the rigidity properties of the corelayer 50 is laminated on the first surface 52 of the core layer 50. Thefirst face sheet 56 may be electrically-conductive, whereupon anelectrically-conductive adhesive 62 may be interposed between the firstface sheet 56 and the first surface 52 of the core layer 50. A secondface sheet, such as second face sheet 58, having rigidity propertiessuperior to the rigidity properties of the core layer 50 may belaminated to the second surface 54 of the core layer 50. If the secondface sheet 58 is electrically conductive, an electrically-conductiveadhesive 64 is interposed between the second face sheet 58 and thesecond surface 54 of the core layer 50.

Many combinations of the components recommended above can be used toachieve a broad range of electromagnetic, thermal, and structuralefficacy. Using the methods described, variously above, it is possibleto build composite structures that will shield from electromagneticsignals that are well in excess of about eighty decibels below afrequency of one megahertz. Such structures have a stiffness of about30×10⁶ pounds per square inch, approximate the stiffness of steel, and adensity of about 1.35 grams per cubic centimeter, less that half thedensity of aluminum. Typical electrical properties achievable include asurface resistivity of about less than or equal to 0.003 ohm-per-squareand a volume resistivity in a range of from about 10⁻² to about 10⁻⁵ohm-centimeters.

By way of recapitulation, the following description of ahigh-performance electromagnetic-shielding system is provided.

An electrically-conductive, and optionally thermally-conductive, porouscore composite panel structure is used as a broadband electromagneticshield. The porous core may be metal foam, a porous self-supportingmetal structure, metal particles, metal nanostrands, or a metal coatedporous substrate, such as an open-cell or closed-cell foam, felt, cloth,fibrous matting, electrically-nonconductive coated particles. Highlyelectromagnetically-permeable metals, such as nickel, copper nickel, andperm alloy are recommended, particularly for the electromagneticshielding from electromagnetic radiation at lower frequencies.

The face sheets used on a foam core need not be electrically-conductive,but more effective electromagnetic shielding is achieved, if the facesheets are electrically-conductive. This is particularly the case, whenit comes to achieving electrical continuity between structurallyadjacent constituent elements in a larger electromagnetic enclosure. Theface sheets may be made conductive by the use of metal wires or meshes,metal coated fibers, conductive resins such as nanostrand bearingresins, and paper nickel-coated by chemical vapor deposition processesapplied to the surface of the face sheets with nickel nanostrand bearingresins. The polymer resins employed in the face sheets range from astiff epoxy to an elastomer. Correspondingly, the resulting compositepanel will be rigid or flexible. The composite face sheets are joined toboth faces of the electrically-conductive porous core with anelectrically-conductive adhesive, such as an adhesive made conductivethrough the inclusion therein of nickel nanostrands.

Electrically-conductive coatings, adhesives, and elastomers are used tojoin panels, achieving mechanical integrity and electrical continuity.Though the porous electrically-conductive core alone is an excellentelectromagnetic shield, electrically-conductive composite face sheets,resins, and adhesives provide significant improvement in the overallelectromagnetic-shielding that can be produced.

In yet another vein, polymer systems and fiber-reinforced compositematerials play increasingly important structural roles in civil,industrial, and military applications. These materials exhibit highspecific stiffness and strength and often have additional advantages inother areas, such as ease of manufacturability. These polymer orcomposite systems exhibit poor electrical properties, particularly withrespect to electromagnetic shielding. A transition is currently takingplace in the materials industry to fabricate structures from fiberreinforced composites, rather than from isotropic metals. An example ofthis process can be observed in aircraft and satellite construction.While such structures have in the past been built of aluminum or otherlightweight metals, it is now increasingly the practice to buildaircraft and satellites of carbon fiber reinforced composites.

Physical and materials considerations of the geometrical morphology ofan electromagnetic shield have importance. A solid, planar shield mustrely on bulk material properties alone, such as conductivity andpermeability, for shielding effectiveness. Several fundamental materialproperties also dictate the electromagnetic capabilities of a material.With respect to electromagnetic shielding, important material propertiesare electrical conductivity, magnetic permeability, thickness, andgeometry. The most significant variable may by each particularapplication, however. It is desirable for any electromagnetic shieldinglayer to offer broadband performance from very low frequencies on theorder of a few kilohertz to very high frequency on the order of tens ofgigahertz. From low to high frequencies, different considerationsdominate depending on the characteristics of the specificelectromagnetic waves involved. Considerations are also significantwhether electromagnetic shielding should be effective in near field orfar field domains. In all cases, it is advantageous to use highlyconductive and highly electromagnetically permeable materials forelectromagnetic shielding applications.

FIG. 4 is an enlarged cross-sectional view of a second embodiment of afront panel 70 incorporating teachings of the present invention for anelectromagnetic-shielding enclosure that resolves these deficiencies incommon composite materials by incorporating teachings of the presentinvention. Front panel 70 includes a structural core 72 having a firstsurface 74 and opposite thereto a second surface 76. The distancebetween first surface 74 and second surface 76 of core 72 defines thethickness T₇₂ of core 72.

A layered, electrically-conducting composite cover 80 is disposed onfirst surface 74 of core 72. Cover 80 has a thickness T₈₀ that mayadvantageously be substantially less than thickness T₇₂ of core 72. Onetechnology found to be useful in constructing electromagnetic shieldingfor fiber reinforced composites is expanded metal foil or metal mesh,either of which may be co-cured directly onto a composite structure, orapplied thereto in a secondary operation. By metal mesh, it isunderstood to mean any sheet-like metal material with porosity,including woven structures and expanded foil structures. The use ofhighly conductivity metals, such as nickel, copper, aluminum, or thealloys thereof are particularly suited to lower frequencyelectromagnetic shielding. Metal foils have excellent electricalconductivity and thus exhibit good electromagnetic shielding protection.The range of protection afforded by these materials is, however, limitedto some extent and in selected situations, by the physical geometry ofthe metal foils. As frequency increases, wavelength decreases. At somepoint a metal mesh may no longer be acceptably effective in shieldingelectromagnetic waves. Still, cover 80 of front panel 70 includes afirst stratum 82 of porous metal that exhibits pronounced low-frequencyelectromagnetic shielding properties. First stratum 82 is secured,either directly or indirectly, as shown in FIG. 4, to first surface 74of core 72.

Another material that has been found to be useful in electromagneticshielding is nickel coated carbon nonwoven broad goods. Nickel isparticularly appropriate at high frequencies, as nickel is highlyelectrically conductive and exhibits high magnetic permeability.Metal-coated non-woven materials are made by forming a nonwoven ofnickel coated carbon fibers or by chemical-vapor-deposition of nickelcoatings on a formed carbon non-woven. The latter are occasionallyreferred to as nickel-coated papers. Metal-coated non-wovens in thiscontext include both non-wovens that are made with metalized fibers andnon-wovens that are metalized in a secondary process. The use of highlyconductivity and high permeability metals, such as nickel and the alloysthereof, are particularly suited to this role, and thechemical-vapor-deposition of these metals is particularly efficient.

Nickel-coated papers provide superior shielding effectiveness with anaccompanying dramatic reduction in weight. The electromagnetic shieldingeffectiveness of a sheet of aluminum, like the electromagnetic shieldingeffectiveness of nickel-coated papers decrease in shieldingeffectiveness at low frequencies, however. Still, a second stratum 84 ofelectrically-conductive elements exhibiting pronounced high-frequencyelectromagnetic-shielding properties is secured in an overlapping andelectrically-continuous relationship to first stratum 82. Typically, theelectrically continuous relationship between the electrically-conductiveelements of second stratum 84 and the porous metal of first stratum 82is effected either by direct contact of those components or through theuse of an electrically conductive adhesive disposed therebetween (notshown in FIG. 4). Second stratum 84 is secured, either directly orindirectly, as shown in FIG. 4, to first surface 74 of core 72. Cover 80may include a protective coating 86 on the outer surface of theoutermost of first stratum 82 and second stratum 84, when cover 80 issecured to core 72.

Thus, front panel 70 is composite electromagnetic shielding technologythat provides broadband shielding, from low frequencies to highfrequencies. A combination of expanded metal foils for low frequencyeffectiveness with metal coated non-woven broad goods for high frequencyeffectiveness results in a compellingly effective synthesis of differentmaterials neither of which alone is capable of providing suitablebroadband shielding. The result in some cases is even superior to theuse of a solid metal sheet. Both first stratum 82 and second stratum 84of cover 80 can be integrated onto core 72 in a single co-curingprocess, if desired, simplifying some aspects of manufacturing acomposite core with effective electromagnetic shielding properties.

Core 72 is structurally self-supporting and may be substantially planaras shown in FIG. 4 with first surface 74 being generally parallel tosecond surface 76 thereof. Core 72 need not be electrically conductive,however. Core 72 may be made of a solid resin or of a body of resin withfibers, such as fibers of glass, carbon, or a polymer embedded therein.

First stratum 82 of cover 80 is a metal lattice, such as a metal screen,a metal mesh, an expanded metal foil, or a perforated metal sheet.Alternatively, the metal lattice of first stratum 82 may be anon-continuous, metal film that is disposed on first surface 74 of core72 and is perforated by a pattern of apertures of dimensionscommensurate with those associated with the forms of metallic latticementioned above. The deposition of such a metal film can be accomplishedusing spraying, vapor deposition, electrodeposition dipping, rolling,and brushing of the metallic material from which that layer is to beconstructed. Whether applied in this manner or disposed as one of theother forms of metal lattice mentioned above, first stratum 82 may bemade from an electromagnetically active metal, such as a μ-metal,nickel, copper, aluminum, bronze, steel, and alloys of any thereof.

The constructions described above will achieve composite structures thatshield very effectively in a frequency range from about 10 kilohertz toabout 100 gigahertz. Typical electrical properties include a surfaceresistivity of about 0.006 ohm-per-square, and a volume resistivity in arange of from about 10⁻² to 10⁻5 ohm-cm.

FIG. 5 is a graph depicting the electromagnetic shielding effectivenessof various qualities of expanded metal having open areas in a range offrom about 60% to about 70%. Five (5) types of expanded metal mesh areshown as tested. For convenience, FIG. 5 includes a highly enlargedillustrative depiction of the pores in such a layer of expanded metal.The electromagnetic shielding effectiveness of each of the samples shownin FIG. 5 is greatest at relatively low electromagnetic frequencies, andthat the electromagnetic shielding effectiveness of each sample declinesseemingly in a linear manner to low levels of electromagnetic shieldingat higher electromagnetic frequencies. This is typical in expandedmetal.

The electrically-conductive elements of second stratum 84 may be anon-woven veil of electrically non-conductive fibers that are coated bya metal. The veil may take the form of a scrim of non-electricallyconductive fibers, a mat of electrically non-conductive fibers, or apaper sheet of electrically non-conductive fibers. The veil can beconstructed from individually-metalized fibers, or the veil may beconstructed of fibers assembled in a non-woven relationship and coatedcollectively with a metal subsequently thereto. Generally the metalcoating will be a chemical vapor deposition of an electromagneticallyactive metal, such as nickel, copper, aluminum, iron, and alloys of anythereof.

FIG. 6 is a graph illustrating the electromagnetic shieldingeffectiveness of chemical-vapor-deposition, nickel-coated, non-wovenfibers of the type suitable for use in second stratum 84 of cover 80.Layers of such material are disposed on the surface of quasi-isotropiccomposite panels made, restrictively, of fiberglass or carbon fiberreinforced resin. The performance of these two (2) forms ofelectromagnetically shielded panels are compared with theelectromagnetic shielding performance of aluminum foil.

FIG. 7 illustrates the synergy produced by the use together of a firststratum of porous metal, such as that employed in first stratum 82 ofcover 80, in an overlapping, electrically-continuous relationship toelectrically conductive elements, such as those used in second stratum84. The porous metal used in first stratum 82 of cover 80 exhibitspronounced low-frequency electromagnetic shielding properties, while theelectrically conductive elements used in second stratum 84 exhibitpronounced high-frequency electromagnetic properties. When metalliclayers of these contrasting types are employed in a single cover in anoverlapping relationship in which electrical conductivity therebetweenis maintained, the electromagnetic shielding effectiveness observed isdramatically broadband and extremely high.

FIG. 8 is a graph depicting to the extent possible the electromagneticshielding effectiveness of a foam metal core of an electromagneticshielding panel of the type depicted in FIG. 2, of electromagneticshielding covers of the type depicted in FIG. 4, and of a baseline ofaluminum foil that is 0.032 inches thick. At lower frequencies, between10⁵ and 10⁶ hertz, the electromagnetic shielding performances of each ofthe constructions tested is discernable to a slight extent. On the otherhand, due to limitations in the test methodology and equipment employedto secure the data exhibited in FIG. 8, in frequency ranges about andabove 10⁶ Hertz, the four (4) performance curves cannot be meaningfullydistinguished one from each other.

The present invention contemplates methods for manufacturing a panel fora broadband electromagnetic shield, such as front panel 70 illustratedin FIG. 4. Such a method commences with the step of forming a core ofsubstantially the same size and shape as the desired electromagneticshielding panel. The core by having first and second surfaces onopposite sides thereof has a thickness defined as the distance betweenthose first and second surfaces. Then, a layered broadbandelectromagnetic shielding cover having a thickness substantially lessthan the thickness of the core is constructed and disposed on the firstsurface of the core. The building of the layered broadbandelectromagnetic shielding cover includes the steps of forming metal intoa porous, first stratum that exhibits pronounced low-frequencyelectromagnetic shielding properties, assembling electrically conductiveelements into a second stratum exhibiting pronounced high-frequencyelectromagnetic shielding properties, and securing the second stratum tothe first stratum in an overlapping electrically-continuousrelationship.

The first stratum is made up of a metallic lattice that may be disposeddirectly on the first surface of the core or secured thereto indirectlyby attachment to the second stratum. The metallic lattice may assumevarious forms, such as a metal screen, a metal mesh, an expanded metalfoil, or a perforated metal sheet. An adhesive may be applied betweenthe first stratum and the first face of the core in securing the coverto the core. In the alternative, the first stratum can be anon-continuous metal film disposed on the first surface of the coreusing, by way of example spraying, vapor deposition, electrodeposition,dipping, rolling, and brushing.

The electrically conductive elements of the second stratum aremaintained in an electrically-continuous relationship with the porousmetal of the first stratum either by direct contact or through the useof an electrically conductive adhesive therebetween. The electricallyconductive elements of the second stratum form a non-woven veil ofelectrically-nonconductive fibers that are coated by a metal. Thosefibers may assume the form of a scrim, a mat, or a paper. The step ofassembling such elements into the second stratum can be conducted byselecting electrically-nonconductive fibers, metalizing individual ofthe fibers to produce individually-metalized fibers, and arraying theindividually-metalized fibers as a non-woven veil. Alternatively, thestep of assembling can include selecting electrically nonconductivefibers, arraying the fibers in a non-woven veil, and metalizing thenon-woven veil.

By way of further reiteration, fiber reinforced composite materialsexhibit highly beneficial physical and mechanical properties withrespect to strength, stiffness, and weight; when compared to the metalsthey often displace. Carbon fiber based composites are, however, poorelectrical conductors, and glass fiber base composites are electricallynonconductive. As a result, composites in general exhibit very poorelectromagnetic properties, and usually require secondary operations toprovide any required electromagnetic protection. According to teachingsof the present invention, such secondary operations include theincorporation into a fiber based composite of metal screens or expandedfoils, surface foils, conductive scrims, conductive resins, andconductive coatings.

The protection provided by these secondary materials is very dependentupon physical and geometrical properties.

For example, metal meshes, expanded foils and screens have been found toprovide acceptable low frequency protection, but as frequencies proceedabove high megahertz or low gigahertz regions, electromagneticwavelengths become short enough to pass right through the pores in thesemetal structures. Conductive coatings or resins have been determined toperform acceptably at higher frequencies, be they filled with materials,such as silver, copper, nickel or carbon particles, or with advancednanomaterials, such as carbon nanotubes and nickel nanostrands. Thesenanomaterials usually provide cost-effective additions of decibels ofprotection, but placing any of these materials in a polymer reduces theconductivity of the consequent composite and results in electromagneticshielding effectiveness that diminishes rapidly at frequencies below 100megahertz. The same is true relative to conductive scrims, non-wovens,and other broad goods. Though shielding well at higher frequencies,these materials do not screen with any particular effectively at lowerfrequencies, and the electromagnetic screening protection provided byconductive broad goods that are woven is greatly reduced at higherfrequencies due to the parallel orientation of the fibers thereof.Carbon fiber composites exhibit a rather narrow range of goodperformance, usually in a range from the high hundreds of megahertz tothe low gigahertz. Above that range, the parallel orientation of thecarbon fibers permits frequency-specific electromagnetic leakage, whilebelow that range, carbon fibers exhibit neither sufficient permittivitynor permeability to provide effective skin depth. Even within theiroptimum range, carbon fiber composites supply all required mechanicalstrengths at thickness that are much less than a thickness needed toprovide meaningful electromagnetic hardening. Metal foils provideacceptable high frequency protection, but metal foils are associatedwith problems, such as a need for significant touch labor, the use ofadhesives that easily degrade, and thermal expansion mismatch problems.Some metals, such as aluminum are corrosive when used as foils withcarbon fiber composites, and ultimately, as already discussed above, asa function of skin depth the electromagnetic screen effectiveness offoils declines at lower frequency.

Amid this swirl of diverse approaches to electromagnetic shielding,there is a frequency range from about 1.0 to about 100 megahertz withinwhich no admirably effective electromagnetic screening can be secured.Above and below this troublesome range, acceptable approaches have beenfound.

Above a range from about 100 to about 500 megahertz, approaches havebeen found that provide remarkable electromagnetic hardening tofrequencies in excess of about 20 gigahertz. By way of example, based oncarbon nanotubes, nickel nanostrands, or metal-coated non-wovens,metallic sheets of less than 2.0 mils in thickness or sprayed metalliccoatings of less than 5.0 mils in thickness have been incorporated intocomposite surfaces and found to produce desired amounts ofelectromagnetic shielding. In so doing, electrical surface resistancewas maintained below 0.1 ohm per square and volume conductance wasmaintained in a range of from about 1000 to about 10,000 Siemens percentimeter.

Below a range of from about 1.0 to about 10 megahertz, metal screens andexpanded foils have been found to provide acceptable electromagneticprotection that approaches the electromagnetic screening effectivenessof a solid metal sheet. Still, above this range, these exhibitelectromagnetic leakage that increases with increasing frequency.

Optimum electromagnetic screening protection can be secured with a boxmade of a metal, such as steel or aluminum. These are, however,relatively heavy construction, particularly if intended to beself-supporting. Prior to the advent of the present invention, acomposite structure that matches the high level of broadband protectionafforded by a metal box has been unattainable.

Employing teachings of the present invention, however, it has been foundthat a nickel-coated, chemical-vapor-deposition, non-woven materialpossessed of acceptable electromagnetic shielding effectiveness down toa range of about 10 megahertz in combination with a copper expanded foilor mesh have together performed as a cover for a reinforced composite offiberglass or carbon fibers that matches the performance of an aluminumplate having a thickness of 0.040 inches (1.0 millimeter). The resultingsurface resistivity of the cover was less than or equal to about 3.0milliohms per square. At higher frequencies, the nickel-coated,chemical-vapor-deposition, non-woven material of the combinationprovided electromagnetic saturation shielding effectiveness to about 20gigahertz. At frequencies below 300 kilohertz, both the covered carbonfiber core and the fiberglass core matched the performance of thespecified aluminum plate, and outperformed the metal foam coretechnology discussed relative to FIG. 2 metal foam core.

The present invention may be embodied in other specific forms withoutdeparting from its structures, methods, or other essentialcharacteristics as broadly described herein and claimed hereinafter. Thedescribed embodiments are to be considered in all respects only asillustrative, and not restrictive. The scope of the invention is,therefore, indicated by the appended claims, rather than by theforegoing description. All changes that come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

What is claimed is:
 1. A method for manufacturing a panel to providereflective broadband electromagnetic shielding, the method comprisingthe steps of: (a) coating a porous nonconductive substrate with ametallic coating to produce a light-weight porouselectrically-conductive material; (b) shaping the light-weight porouselectrically-conductive material into a substantially planar core layerhaving first and second surfaces on opposite sides thereof; and (c)laminating a first face sheet to the first surface of the core layer,the first face sheet having rigidity properties superior to the rigidityproperties of the core layer, wherein a range of frequencies shielded isa function of skin depth of the panel.
 2. The method as recited in claim1, wherein the porous nonconductive substrate is selected from the groupconsisting of nonwoven fibrous matting, paper, and open-cell nonmetallicfoam.
 3. The method as recited in claim 1, wherein the metallic coatingcomprises chemical vapor deposition of an electromagnetically activemetal.
 4. The method as recited in claim 3, wherein theelectromagnetically active metal is selected from the group consistingof nickel, copper, aluminum, iron, and alloys thereof.
 5. The method asrecited in claim 1, wherein the metal coating comprises nickel.
 6. Themethod as recited in claim 1, wherein the first face sheet iselectrically-conductive, and the step of laminating the first face sheetcomprises the step of interposing an electrically-conductive adhesivebetween the first face sheet and the first surface of the core layer. 7.The method as recited in claim 1, further comprising the step oflaminating a second face sheet to the second surface of the core layer,the second face sheet having rigidity properties superior to therigidity properties of the core layer.
 8. The method as recited in claim7, wherein the second face sheet is electrically-conductive, and thestep of laminating the second face sheet comprises the step ofinterposing an electrically-conductive adhesive between the second facesheet and the second surface of the core layer.
 9. A method as recitedin claim 1, wherein the metallic coating comprises metallic nanostrands.